1. Field of the Invention
The invention relates to a superconductive electrical cable which includes at least one superconductive conductor composed of strips or wires that are wound around a support constructed as a pipe in at least one layer (WO 03/052775 A1).
2. Description of Related Art
In today's technology, a superconductive cable includes electrical conductors of a composite material which contains ceramic material which changes over into the superconductive state at sufficiently low temperatures. The electrical direct current resistance of an appropriately constructed conductor is zero when sufficiently cooled, as long as a certain current strength, the critical current strength is not exceeded. Suitable ceramic materials are, for example, oxidic materials on the basis of rare earths (ReBCO), particularly YBCO (yttrium-barium-copper oxide), or BSCCO (bismuth-strontium-calcium-copper oxide). Sufficiently low temperatures for bringing such a material into the superconducting state are, for example, between 67K and 110K. Suitable cooling agents are, for example, nitrogen, helium, neon or mixtures of these materials.
The known cable according to the above-mentioned WO 03/052775 A1 includes a superconductive conductor which is shaped in at least one layer around a pipe. The cable further. includes layers surrounding the conductor. It is arranged by leaving a free space in a cryostat composed of two metal pipes which are arranged coaxially relative to each other in between which a vacuum insulation is provided. A cooling agent for effecting the superconductive state of the conductor can be conducted through the pipe and through the free space of the cryostat.
The conductor of the superconductive cable is composed in today's technology of strips or wires of superconductive material which are wound in at least one layer around a support, for example, a pipe. By cooling the conductor as required. for operating the cable from room temperature to the temperature needed for the superconductive state, the superconductive conductor material shrinks by about 0.25% to 0.3%. In a cable length of, for example, 600 m, this may lead to a shortening of the conductor by about 1.5 m to 1.8 m, The superconductive cable and, thus, also its conductor is secured at its ends in connection fittings. The significant shortening of the conductor due to cooling leads to a substantial tensile load of the connect on fittings. In addition, it may easily lead to an overextension of the conductor or its individual elements and may lead to damage as a result which makes the conductor useless. For preventing such effects on the operability of the conductor, in accordance with EP 1 821 380 B1, its ends are to be fixed within the cryostat in a length which is shortened corresponding to the superconductive state only after cooling has taken place. This measure is relatively complicated. For example, when such a superconductive cable is heated, for example, for repair purposes to room temperature, the connecting fittings are additionally subjected to mechanical load due to the expanding cable.